In recent years, there has been a growing tendency toward the increase of the size of semiconductor wafer and the reduction of the thickness of wafer for IC card. In order to satisfy the requirements for ease of peeling the protective tape for working the wafer, there have been used more radiation-curing protective sheets. The radiation-curing protective sheet can be easily peeled but is disadvantageous in that when irradiated with radiation, the adhesive itself undergoes shrinkage to produce a shrinkage force that causes the wafer to be warped, making it difficult for the wafer to be conveyed at the production process.
The radiation-curing protective sheet also gives a problem at so-called dicing step for picking up semiconductor elements which have been cut and separated. In other words, when an adhesive sheet made of an adhesive which is liable to great volume shrinkage is used, the sheet cannot be sufficiently expanded, making it impossible to expand the dicing street and hence causing troubles in picking up the semiconductor elements.
In general, a radiation-curing adhesive is prepared by using a high molecular weight compound called base polymer (main polymer), a radiation-polymerizable compound (radiation-reactive oligomer, etc.) having a weight-average molecular weight of 20,000 or less and a carbon-carbon double bond in its molecule and a radiation polymerization initiator as essential components, and appropriately added various additives such as crosslinking agent thereto. In order to provide the adhesive with characteristics that the adhesion is greatly lowered after irradiation with radiation, a so-called polyfunctional compound having two or more carbon-carbon double bonds per molecule is widely used as the radiation-polymerizable compound. When such a radiation-curing adhesive is irradiated with radiation, the radiation-polymerizable compound undergoes reaction to form a three-dimensional network structure rapidly, causing the entire adhesive to react and harden rapidly and show a drop of adhesion. These techniques are disclosed in, for example, JP-A-5-32946 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-8-20756, JP-A-9-186121, JP-A-9-298173 and JP-A-11-26406. However, this reaction/curing is accompanied by great volume shrinkage of the adhesive that causes the development of shrinkage stress resulting in the above-described disadvantages.
It can be proposed that a radiation-polymerizable compound having as few functional groups as possible be used to overcome these disadvantages. However, this approach is disadvantageous in that the resulting adhesive cannot show a sufficient drop of adhesion after irradiation with radiation and thus loses inherent performances of an adhesive.
The following method is proposed as a method for reducing the shrinkage force developed by curing reaction caused by irradiation with radiation: a method of decreasing the amount of the radiation-reactive oligomer to the main polymer; specifically, a method which comprises mixing about 10 to 40 parts by weight of the radiation-reactive oligomer with 100 parts by weight of the main polymer. In this method, it is proper that an oligomer which allows sufficient drop of adhesion even when used in a small amount, e.g., polyfunctional oligomer having 6 or more unsaturated bonds, be selected. In order to maintain a desired adhesion before irradiation with radiation, it is preferred that an acrylic acid ester copolymer having a low elastic modulus be selected as a main polymer. Another possible method for reducing the shrinkage force is a method which involves the selection of a compound having 1 to 4 unsaturated bonds per molecule which undergoes low shrinkage as a radiation-reactive oligomer while mixing about 30 to 300 parts by weight of the radiation-reactive oligomer with 100 parts by weight of the main polymer. In this case, it is preferred that an oligomer which undergoes low shrinkage but shows a reduced adhesion be selected. Further, a method is proposed which comprises mixing a mixture of about 20 to 300 parts by weight of a polyfunctional oligomer having 6 or more unsaturated bonds and a low-functional oligomer having from 1 to 4 unsaturated bonds per molecule with 100 parts by weight of the main polymer.
However, according to the inventors' studies, it has been found difficult in all the above approaches to satisfy both the two requirements for sufficient drop of adhesion and low shrinkage force of adhesive at the same time. One of the reasons for the above difficulty is that complete compatibility is made difficult in such a system having a main polymer and a radiation-reactive oligomer in admixture. In order to accomplish sufficient drop of adhesion, excess amount of oligomer is required. When excess amount of oligomer is used, a high shrinkage force is developed.